Are FMIC upgrades a noticeable upgrade? I know it lowers your BATs, but will that alone improve performance? Or do you need a tune that raises the PSI a bit? (I'm interested in joining this GB, but I'm on the fence about whether this is a noticeable upgrade?)

To take full advantage of a FMIC upgrade, you would need a tune. That said, when installed without a tune, you will still see a noticeable power gain. Cooler air and less restriction will always make more power.